The rotational speed of a gas turbine engine, particularly the rotational speed of the high pressure spool shaft of the engine (sometimes referred to as the N2 speed), is a primary input variable necessary for the control logic of a gas turbine engine. In the prior art, engine speed is detected by way of a sensor positioned adjacent to a phonic wheel which is usually incorporated at a suitable location along a rotor of the engine, such as the high pressure spool shaft. A phonic wheel typically defines a number of slots extending therethrough and is mounted on a rotor shaft. A medium such as a beam of light, a magnetic field, etc. is employed such that the sensor receives the medium affected by the slots of the phonic wheel when rotating, thereby enabling it to provide data regarding the rotational speed of the rotor shaft. The phonic wheel and the associated sensor are conventionally buried within the engine, which makes access thereto for maintenance and repair very difficult. Furthermore, the conventional location of the phonic wheel and associated sensor of a gas turbine engine is in a high temperature environment inside of the engine and this can cause a high differential thermal expansion mismatch between the sensor and the tips of the phonic wheel.
Accordingly, there is a need to provide an improved method and apparatus for detecting the rotational speed of gas turbine engines.